Many metal complexes are able to react cleanly with singlet oxygen to make new metal-peroxo complexes. This methodology suggests a new approach toward the preparation of metal-dioxygen complexes, leading to new model systems that mimic important enzymes and to new insights into the mechanism of oxygen atom transfer to a variety of substrates. Reactions of transition metal complexes with singlet oxygen can be carried out at very low temperatures. Thus, thermodynamically very unstable metal-dioxygen complexes whose existence has previously been limited to the realm of speculation can be synthesized, directly observed, and their reactivities can be studied. At the same time, the scope of oxidative addition of singlet oxygen to metals will be studied. Thus far, no such reactions have been reported for early transition metals. We will attempt to prepare several biologically relevant peroxo complexes derived from early transition metals with singlet oxygen. Other subprojects will probe the mechanistic features of the oxidation of metal-coordinated thiolates. It will be examined whether or not oxidation of the metal followed by oxygen atom transfer to the sulfur atom is a viable reaction pathway. Another subproject involves the synthesis of an endoperoxide coordinate to a copper complex as a model for the copper enzyme quercetinase. Different reactions of singlet oxygen with dinuclear complexes will also be examined. In summary, the various rather diverse subprojects reflect the diverse areas in which the utilization of singlet oxygen as a mechanistic tool may lead to important mechanistic insights.